Which bacterium is a key starter culture for making yogurt?

Which bacterium is a key starter culture for making yogurt? Lactobacillus bulgaricus is one of yogurt’s two defining starter bacteria.

Its current scientific name is Lactobacillus delbrueckii subsp. bulgaricus. It is a Gram-positive, rod-shaped lactic-acid bacterium that ferments lactose in milk into lactic acid. The acid lowers milk’s pH, causes casein proteins to coagulate, and gives yogurt its thick texture and characteristic tang.

Yogurt production usually relies on a partnership between L. bulgaricus and Streptococcus thermophilus. S. thermophilus grows rapidly early in fermentation, while L. bulgaricus breaks down milk proteins and releases compounds that support its partner. Together they also produce acetaldehyde, a major contributor to yogurt’s aroma.

A common mix-up is treating L. bulgaricus as the only yogurt bacterium. Traditional yogurt cultures generally contain both organisms, and some commercial products add other lactobacilli or bifidobacteria. The name “bulgaricus” reflects the bacterium’s association with Bulgarian fermented milk traditions; the organism was formally described by Stamen Grigorov in 1905.